Strain wave gearings are often lubricated with an oil or grease. A strain wave gearing disclosed in Patent document 1 is provided with a mechanism in which a grease is supplied from a grease reservoir to portions where lubrication is required. In a strain wave gearing disclosed in Patent document 2, an oil tank is mounted inside a cup-shaped externally toothed gear so as to rotate integrally with the externally toothed gear, whereby making use of centrifugal force to supply an oil from the oil tank to portions where lubrication is required. In a power transmission device disclosed in Patent document 3, for the purpose of reducing friction and wear between sliding members, the sliding surfaces of these member are set to be a surface having a prescribed surface roughness and being covered with a carbon based film, and additionally a grease is employed to lubricate between the sliding surfaces covered with the carbon-based films.